Fluid Coupling Overview and Applications

Fluid Coupling Overview
  A fluid coupling contains three components, plus the hydraulic fluid:
  The housing, also called the shell (which will need to have an oil-restricted seal around the travel shafts), provides the fluid and turbines.
  Two turbines (fan like components):
  One connected to the insight shaft; known as the pump or impellor, primary steering wheel input turbine
  The other linked to the output shaft, referred to as the turbine, result turbine, secondary steering wheel or runner
  The traveling turbine, referred to as the ‘pump’, (or driving torus) is usually rotated by the prime mover, which is normally an interior combustion engine or electric engine. The impellor’s motion imparts both outwards linear and rotational movement to the fluid.
  The hydraulic fluid is directed by the ‘pump’ whose shape forces the stream in the direction of the ‘output turbine’ (or powered torus). Right here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ lead to a net push on the ‘output turbine’ leading to a torque; hence causing it to rotate in the same path as the pump.
  The motion of the fluid is efficiently toroidal – exploring in one path on paths which can be visualised as being on the top of a torus:
  If there is a difference between input and result angular velocities the movement has a component which is definitely circular (i.e. round the bands formed by parts of the torus)
  If the insight and output phases have similar angular velocities there is no net centripetal push – and the movement of the fluid is circular and co-axial with the axis of rotation (i.e. across the edges of a torus), there is absolutely no stream of fluid in one turbine to the various other.
  Stall speed
  A significant characteristic of a fluid coupling is its stall acceleration. The stall velocity is defined as the best speed of which the pump can change when the result turbine can be locked and optimum input power is used. Under stall circumstances all of the engine’s power would be dissipated in the fluid coupling as heat, possibly leading to damage.
  Step-circuit coupling
  A modification to the easy fluid coupling is the step-circuit coupling that was formerly manufactured as the “STC coupling” by the Fluidrive Engineering Firm.
  The STC coupling includes a reservoir to which some, however, not all, of the essential oil gravitates when the result shaft is normally stalled. This decreases the “drag” on the input shaft, resulting in reduced fuel consumption when idling and a reduction in the vehicle’s tendency to “creep”.
  When the output shaft starts to rotate, the essential oil is thrown out of the reservoir by centrifugal pressure, and returns to the main body of the coupling, to ensure that normal power transmitting is restored.
  Slip
  A fluid coupling cannot develop output torque when the input and output angular velocities are identical. Hence a fluid coupling cannot achieve completely power transmission performance. Due to slippage that will occur in virtually any fluid coupling under load, some power will always be dropped in fluid friction and turbulence, and dissipated as heat. Like other fluid dynamical gadgets, its efficiency will increase steadily with increasing scale, as measured by the Reynolds amount.
  Hydraulic fluid
  As a fluid coupling operates kinetically, low viscosity liquids are preferred. In most cases, multi-grade motor natural oils or automatic transmission liquids are used. Increasing density of the fluid escalates the quantity of torque that can be transmitted at a given input speed. Nevertheless, hydraulic fluids, much like other fluids, are at the mercy of changes in viscosity with heat change. This leads to a change in transmission functionality therefore where unwanted performance/efficiency change needs to be held to a minimum, a motor essential oil or automated transmission fluid, with a higher viscosity index ought to be used.
  Hydrodynamic braking
  Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as heat through frictional forces (both viscous and fluid/container). Whenever a fluid coupling is used for braking it is also referred to as a retarder.

Fluid Coupling Applications
  Industrial
  Fluid couplings are used in many commercial application concerning rotational power, specifically in machine drives that involve high-inertia begins or continuous cyclic loading.
  Rail transportation
  Fluid couplings are found in a few Diesel locomotives within the power transmitting system. Self-Changing Gears produced semi-automated transmissions for British Rail, and Voith manufacture turbo-transmissions for railcars and diesel multiple products which contain various combinations of fluid couplings and torque converters.
  Automotive
  Fluid couplings were used in a number of early semi-automated transmissions and automated transmissions. Since the late 1940s, the hydrodynamic torque converter has replaced the fluid coupling in automotive applications.
  In automotive applications, the pump typically is connected to the flywheel of the engine-in reality, the coupling’s enclosure may be area of the flywheel correct, and therefore is turned by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmitting is in equipment, as engine swiftness increases torque can be transferred from the engine to the input shaft by the motion of the fluid, propelling the automobile. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch traveling a manual transmitting.
  Fluid flywheels, as distinct from torque converters, are best known for their use in Daimler vehicles in conjunction with a Wilson pre-selector gearbox. Daimler utilized these throughout their selection of luxury cars, until switching to automated gearboxes with the 1958 Majestic. Daimler and Alvis were both also known for their military vehicles and armored vehicles, some of which also used the combination of pre-selector gearbox and fluid flywheel.
  Aviation
  The many prominent utilization of fluid couplings in aeronautical applications was in the DB 601, DB 603 and DB 605 motors where it was utilized as a barometrically managed hydraulic clutch for the centrifugal compressor and the Wright turbo-substance reciprocating engine, where three power recovery turbines extracted approximately 20 percent of the energy or about 500 horsepower (370 kW) from the engine’s exhaust gases and, using three fluid couplings and gearing, converted low-torque high-acceleration turbine rotation to low-speed, high-torque result to drive the propeller.

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